rt2800: 5592: add AGC init

Based on:
RT5592_RTMPAGCInit()

from:
DPO_RT5572_LinuxSTA_2.6.1.3_20121022/chips/rt5592.c

Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Tested-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2800lib.c

@@ -2658,7 +2658,10 @@ static void rt2800_config_channel(struct rt2x00_dev *rt2x00dev,
 		rt2800_bbp_write(rt2x00dev, 195, 141);
 		rt2800_bbp_write(rt2x00dev, 196, conf_is_ht40(conf) ? 0x10 : 0x1a);
 
-		/* TODO AGC adjust */
+		/* AGC init */
+		reg = (rf->channel <= 14 ? 0x1c : 0x24) + 2 * rt2x00dev->lna_gain;
+		rt2800_bbp_write_with_rx_chain(rt2x00dev, 66, reg);
+
 		rt2800_iq_calibrate(rt2x00dev, rf->channel);
 	}